Hydraulic circuits for controlling a reversible hydraulic motor typically include a pump for circulating a hydraulic fluid, various conduits, and numerous valves. Some valves which are employed in such circuits include a three-position four-way directional control valve having a single spool for controlling fluid flow from the pump to the motor and from the motor to a tank, a pair of line reliefs operatively associated with opposite sides of the reversible hydraulic motor, one or more counterbalance valves, load check valves to block reversible flow of fluid if the load pressure is higher than the pump pressure at the time the directional control valve is shifted, and make-up valves. One problem encountered with such circuits is that there is no way to avoid or compensate for an overspeed condition of the hydraulic motor. For example, an operator may want to have the hydraulic motor operate at a preselected speed. However, the pump, which supplies hydraulic fluid to the motor, may cause the motor to go faster than the preselected speed. In such a case, there should be some mechanism associated with either the hydraulic circuit or the motor for braking or slowing the motor down to the preselected speed.
In view of the above, it would be desirable to provide a control system for detecting an overspeed condition and for braking the hydraulic motor once an overspeed condition is detected or occurs. Further, it would be advantageous to remove some of the hydraulic fluid from the hydraulic circuit to be used by the pump at a later time. It would also be desirable to construct such a control system which minimizes the number of valves that need to be used to reduce the cost associated with construction and the time required to develop such control systems.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.